PerlESA 2009/2010

Eelco Schatborneelco@os3.nl

1 October 2009

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ESA: Perl

Today:

1. Perl introduction

2. Basic Perl: types, variables, statements, . . .

3. Object Oriented Perl

4. Documentation

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Perl Introduction

I “Practical Extraction and Report Language”?

I Developed by Larry Wall, as systemadministrator at NASA, in the late 1980s

I Created as a way to make report processingeasier.

I It’s an interpreted language, but can becompiled as well

I Is now being used for:I System administration automationI Glue between systems, conversionI CGI backend for websitesI Much more. . .

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Simple Perl

A simple program to start with:

#!/usr/bin/perlprint "Hi there!\n";print "This is a very";print "\tsimple program.\n";

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Numbers

simple numbers as per usual:

I decimal: 12, -17, 255 . . .

I octal, start with 0: 015, -023, 0777 . . .

I hexadecimal, start with 0x: 0xc, -0x11, 0XfF, . . .either case allowed for the x or hex digits abcdef.

floating point numbers:

I “one and a quarter”: 1.25

I “7.25 times 10 to the 45th power”: 7.25e45.

I “negative 12 times 10 to the -24th”: -12e-24.

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Strings

I Escape sequences represent special characters:\n, \t, . . .

I Only in double quoted (‘"’) strings

I For verbatim strings use single quotes (‘’’)

Perl: print "Hi there! \n \t It’s me.\n";print ’And me \n \t as well!’;

output: Hi there!It’s me.

And me \n \t as well!

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Variables

I Three different types of variables:scalars, arrays and hashes

I Start with the dollar (‘$’), at (‘@’) or percentage (‘%’) sign,depending on the type of variable

I Variable names are a punctuation character, a letter orunderscore, and one or more alphanumeric characters orunderscores.

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Scalar variables

I Start with the dollar (‘$’) sign.

I Can be a number or a string (or an object reference)

I The usual operators apply:

+, -, *, /, =, +=, -=, *=, /=, ++, --

I Perl automatically converts between numbers and strings.

$n = 8 + "2" → $n holds "10"

I But concatenation of strings using a period ‘.’

$n = 8 . "2" → $n holds "82"

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Array variables (1)

I Start with the at (‘@’) sign.

I Arrays represent ordered lists of scalars.

I Definition using parenthesized, comma delimited lists:@numbers = ( 1, 2, 4, 8, 16 );@strings = ( "one", "two", "3" );

I The index of an array starts at 0, not 1

I Processing single items as a scalar using ‘[’ and ‘]’:print $strings[1]; → prints "two"$strings[1] = "2"; → we changed @strings

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Array variables (2)

I Arrays can be created using scalar assignment:$names[0] = "Eelco";→ creates @names to hold ("Eelco")

I The length of an array: $#strings, returns the number ofitems in the array minus one. If equal to -1, the array doesnot exist or is empty.

print $#strings; → prints 2

I Assignment to $#strings changes the size of the array.$#strings = 0;→ @strings now holds ( "one" )

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Hash variables (1)

I Start with the percentage (‘%’) sign.

I Made up of keys and values.

I Each key has exactly one corresponding value.

I Definition using parenthesized, comma delimited lists of keysand values:

%numbers = ( "one" => "one", "two" => 2 );

I Processing a single hash as a scalar using ‘{’ and ‘}’:print $numbers{"two"}; → prints "2"$numbers{"one"} = "1"; → we changed %numbers

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Hash variables (2)

I Like arrays, hashes can be created using scalar assignment:$strings{"1"} = "one";→ creates %strings to hold ( "1" => "one" )

I The keys <hash> function will return an array filled with allthe keys in the hash.

print keys %numbers; → prints ( "one", "two" )

Question: How do ascertain the number of items in ahash?

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Flow control

I A block of code contain a list of statements.

I Code blocks are delimited by ‘{’ and ‘}’

I A perl program is also a block of code.

I Flow control can be excerted using looping or conditionalstatements.

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Looping Statements

I For loops are for repetition

I Using a list and an index variable a block of code will beexecuted for each value in the list.

for $i ( 1, 2, 3, 4 ) { . . . }

I Using ‘..’ you can also give a range:for $i ( 1 .. 3, 4 ) { . . . }

I The list can contain any scalars or other lists.@range = ( 1 .. 3 );for $i ( @range, 4 ) { . . . }

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Conditional Statements (1)

I Conditional statements:if ( test ) { . . . } elsif { . . . } else { . . . }unless ( test ) { . . . } else { . . . }while ( test ) { . . . }until ( test ) { . . . }

I The usual comparison operators for testing:<, >, ==, !=, <=, >=

I But comparison of strings using ‘eq’.0xa == " 10 " → true0xa eq " 10 " → false

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Conditional Statements (2)

if ($a == 5) {print "It’s five!\n";

} elsif ($a == 6) {print "It’s six!\n";

} else {print "It’s something else.\n";

}

unless ($pie eq ’apple’) {print "Ew, I don’t like $pie flavored pie.\n";

} else {print "Apple! My favorite!\n";

}

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Conditional Statements (3)

$a = 0;

while ($a != 3) {$a++;print "Counting up to $a...\n";

}

until ($a == 0) {$a--;print "Counting down to $a...\n";

}

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Perl Regular expressions (1)

I Very mature, can be highly complex

I Can be used for matching (testing) or transliteration.

I Simple syntax is / . . . /

I The matching operator ‘=~’ is used for testing

Perl: if ("This is perl!" =~ /is/) {print "Match!";

}output: Match!

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Perl Regular expressions (2)

Jaap already treated most of this:

I ‘^’ and ‘$’ match the beginning and end of a stringrespectively.

I You can use character classes (‘[’ . . . ‘]’).

I You can quantify matches using ‘*’, ‘+’ or ‘?’ after a characteror character class.

I You can quantify matches generically using ‘{ from , to }’.

I ‘.’ matches any character.

I Use a backslash (‘\’) to escape characters like:‘.’, ‘^’, ‘$’, ‘/’, ‘\’, ‘{’, ‘}’, ‘[’, ‘]’, . . . .

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Perl Regular expressions (3)

Metacharacters in regular expressions:

\d Matches a single digit character\w Matches a single word character (a letter, digit or underscore)\s Matches a whitespace character. . . . . .

Flags that can follow a regular expression:

i Match case insensitivelyg Remember the current position for the next matching

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Perl Regular expressions (4)

Matching subexpressions:

I You can match subexpressions by encapsulating themwith ‘(’ and ‘)’

I Each subexpression is stored in a variable $1 . . . $n, which canbe used later on.

Perl: "This is it!" =~ /\s(..)/;print $1;

Question: What is printed by the above lines?

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Perl Regular expressions (4)

Search and replace:

I Using s/ regexp / replacement /

Perl: $a = "This is it, or is it?"$a =~ s/\s.(s)/ wa$1/;print $a;

output: This was it, or is it?

I Using the g flag the replacement will be made for all matches,instead of just the first one.

Perl: $a = "This is it, or is it?"$a =~ s/\s.(s)/ wa$1/g;print $a;

output: This was it, or was it?

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Subroutines

I Subs are functions, they can have any arguments

I The arguments are handed down using the special array @_

I A value can be returned using return

Perl: sub multiply {my (@ops) = @_;return $ops[0] * $ops[1];

}multiply(2, 3);

output: 6

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Packages (1)

I Packages are a way to split up perl code into several pieces

I A package can be included into a piece of code by using use

I Each package represents a new namespace

I Create a package by creating a new file, generally ending in.pm

I The code for the class you are creating goes in there.

I The classname often is the same as the filename, this is easierto use, but not obligatory.

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Packages (2)

Create a new file, for instance MyPackage.pm

I The first line should provide the name of the class in thepackage:

package MyPackage;

I Next comes the code for the package . . .

I The file should end with 1; to indicate to the loading modulethat the entire package was read successfully.

Example:

package MyPackage;print "WOW, it worked!\n";1;

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Packages (3)

To use our package run the following perl code:

#!/usr/bin/perluse MyPackage;

The output of the program:

WOW, it worked!

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Packages (4)

I You can refer to variables and filehandles in other packages byprefixing the identifier with the package name and a doublecolon:

$Package::Variable

I You can create subs in a package in the normal way

I They can be called from the program using the package:$Package::the_sub("well?")

I Special code blocks can be used for initialization anddestruction:

BEGIN {}, END {}, INIT {}, CHECK {}

Use perldoc perlmod to get more information

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Objects (1)

“An Object is simply a reference that happens to knowwhich class it belongs to.”

I No special syntax for constructors, you have to make your own

I An object is created using the bless()

I The bless() function creates a reference to an object

I Anything can become blessed

I bless {} allocates an anonymous, empty hash and returns areference to it

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Objects (2)

Using an anonymous reference:

package Critter;sub new { bless {} }

Using anonymous objects the class will not know itself, using aknown reference:

package Critter;sub new {my $self = {};bless $self;return $self;

}

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Classes (1)

“A class is simply a package that happens to providemethods to deal with object references.”

I A class provides the methods for dealing with its reference

I Construction by defining a constructor function, generallynew()

package Critter;sub new {my $self = {};bless $self;return $self;

}

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Classes (2)

I A method is simply a subroutine that expects an objectreference (or a package name, for class methods) as the firstargument.

I A constructor can call methods using the reference

sub new {my $self = {};bless $self;$self->initialize();return $self;

}

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Classes (3)

I Classes can be inherited

I Use the two argument version of bless()

I Bless an object into a class

sub new {my $class = shift;my $self = {};bless $self, $class;$self->initialize();return $self;

}

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Documentation

I BOOKS!, they are in the back of the classroom. . .

I Use the web, there are a lot of websites on perl

I Check www.perl.com for help.

I Find out about perldoc

Material for these slides was taken fromhttp://www.perl.com/pub/a/2000/10/begperl1.html

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